Adaptive responses of rats trained with reductions in exercise heart rate.

This study was designed to test the hypothesis that rats trained with marked reductions in exercise heart rate respond with adaptations indicative of increased intrinsic myocardial performance. Therefore, we measured changes in maximum work capacity (VO2max), biochemical-functional indexes of cardiac contractile capacity, and skeletal muscle oxidative capacity in normal-trained (NT) rats and in rats trained while receiving the selective cardiac beta 1-blocking drug atenolol (AT). Training consisted of treadmill running at approximately 80% VO2max (untrained) for 1-h duration, 6 days/wk, for a total of 8 wk. Exercise heart rate of the AT group was markedly reduced and averaged 140 beats/min below the NT group for any given session. Compared with sedentary controls, VO2max was increased by 11%, and red vastus lateralis muscle citrate synthase activity was increased by 28% in both AT and NT groups (P less than 0.05). There were no differences between trained and nontrained groups with regard to Ca2+-regulated myofibril adenosinetriphosphatase. In situ derived left ventricular pressure and the maximum rate of left ventricular pressure development were not augmented relative to sedentary control values when the trained hearts were either stimulated inotropically or maximally afterloaded . These findings suggest that maximum exercise capacity can be enhanced in rodents conditioned with and without limited elevation in exercise heart rate; however, this reduction of exercise heart rate acceleration does not provide a stimulus to enhance the intrinsic functional capacity of the rodent heart.